This invention relates generally to gyroscope compasses and more particularly to a single degree of freedom gyrocompass driven by a permanent magnet motor and employing gyro rotor speed modulation techniques to determine the direction of true north.
The determination of true north using gyroscopes--gyrocompassing--is well known. In one method of gyrocompassing, a single degree of freedom gyroscope is arranged with its output axis aligned with the vertical and its input axis in the horizontal plane, pointing approximately in the east-west direction. The torque required to maintain the gyro rotor at a null orientation is measured and recorded. The gyroscope rotor is then rotated precisely 180.degree. about the output axis and the torque again measured and recorded. In this way, the effects of inherent gyro drift are differentiated from earth rate effects, allowing heading to be determined.
Another method of gyrocompassing using a single degree of freedom gyro and rotor speed modulation techniques is also well known in the gyrocompassing art. A single degree of freedom gyro is mounted in a gimbal so that the gyro rotor, in addition to its freedom to spin about its spin axis, is free to rotate about a single axis perpendicular to the rotor spin axis. This axis is known as the output axis. For gyrocompassing with a single degree of freedom gyroscope, a torquing means, usually electromagnetic, applies torque about the output axis so as to maintain the rotor at a null angular orientation. That is, the applied torque is made equal in magnitude and opposite in direction to other torques occurring about the output axis. These other torques may be modeled as the sum of a torque due to the coupling of a component of the earth's angular velocity and nonspecific torques which lead to gyro drift. The nonspecific torques have myriad origins: electromagnetic disturbances, mechanical and dynamic imbalances and thermally caused distortions, to mention just a few.
Gyrocompassing with a single degree of freedom gyro using wheel speed modulation entails operating the gyro at a reference rotor speed and recording the torque required to maintain null rotor orientation. This measured torque is thus equal in magnitude to the disturbing torques, one component of which is that due to the earth's angular velocity. Importantly, that torque arising from the earth's rotation is directly proportional to the gyro rotor's angular momentum, and for a rotor with a constant moment of inertia, to the rotor's angular velocity. At a different rotor speed, therefore, the earth's rotation will cause a different disturbing torque. If it be assumed that the disturbing torques other than that due to earth rotation remain constant (or change in predictable ways) with changes in rotor speed then it is possible to determine the contribution of earth rotation to the total disturbing torques about the gyro output axis. From the magnitude of this earth rotation-induced disturbing torque can be calculated the direction of the earth's angular velocity vector, true north.
Gyrocompassing by means of rotor speed modulation has achieved but moderate accuracy in the past because of unpredictable changes in the disturbing torques (other than earth rotation induced) with rotor speed changes. A major source of this unpredictability has been the inconsistency in the magnetization level achieved within hysteresis motors, commonly used to drive gyro rotors. Each time the rotor is brought into synchronization at a different speed, a slightly magnetization of the hysteresis material occurs. This in turn causes different motor efficiencies and changes in the current level in the stator windings. The varying efficiencies cause changes in thermally generated torques and also changes in the motor magnetic field. This field interacts with other magnetic fields such as that of the earth and with conductive and permeable materials located near the torque summing member to produce disturbing torques about the gyro output axis which vary in unpredictable ways with rotor speed changes.
It is an object of the present invention, therefore, to provide an apparatus and method for determining true north very accurately using rotor speed modulation techniques.